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Re: Querying distinct values from a large table

From: Bruno Wolff III <bruno(at)wolff(dot)to>
To: Igor Lobanov <ilobanov(at)swsoft(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Querying distinct values from a large table
Date: 2007-01-30 16:44:35
Message-ID: 20070130164435.GA22099@wolff.to (view raw or flat)
Thread:
Lists: pgsql-performance
On Tue, Jan 30, 2007 at 14:33:34 +0600,
  Igor Lobanov <ilobanov(at)swsoft(dot)com> wrote:
> Greetings!
> 
> I have rather large table with about 5 millions of rows and a dozen of 
> columns. Let's suppose that columns are named 'a', 'b', 'c' etc. I need 
> to query distinct pairs of ('a';'b') from this table.
> 
> Is there any way to somehow improve the performance of this operation? 
> Table can not be changed.

DISTINCT currently can't use a hash aggregate plan and will use a sort.
If there aren't many distinct values, the hash aggregate plan will run much
faster. To get around this limitation, rewrite the query as a group by.
Something like:
SELECT a, b FROM table GROUP BY a, b;

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Subject: Re: Querying distinct values from a large table
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